1. Field of the Invention
The present invention generally relates to electrophotographic printing machines, and more particularly to an electrophotographic printing machine equipped with an image fixing device provided in a carriage performing printing.
Recently, electrophotographic printing machines having a carriage operating in an electrophotographic process have been developed due to demands for cost reduction and down-sizing of electrophotographic recording machines. Generally, such electrophotographic printing machines operate so that a carriage is moved on a transfer device in the direction traverse (perpendicular) to the direction in which paper is transported, and an image transferred on the paper is fixed by means of a roller-shaped fixing device disposed in the paper transporting direction. In order to facilitate down-sizing, an electrophotographic printing machine has been developed in which the fixing device is provided in the carriage. Nowadays, it is required that such an electrophotographic printing machine operates safely and the size thereof is further reduced.
2. Description of the Prior Art
FIGS. 1A, 1B and 1C show the structure of a carriage provided in a conventional electrophotographic printing machine. More particularly, FIG. 1A is a plan view of the carriage and its peripheral parts, FIG. 1B is a cross-sectional view taken along a line I.sub.B --I.sub.B shown in FIG. 1A, and FIG. 1C is a cross-sectional view taken along a line I.sub.C --I.sub.C shown in FIG. 1A.
Referring to FIGS. 1A through 1C, a carriage 11 is made up of a process unit 12 and a fixing device 13, and is moved, by means of a driving motor (not shown for the sake of simplicity) above a transfer unit (print platen) 15 along guides 14a and 14b in the direction traverse to the paper transporting direction. On the either side of the carriage 11, transporting rollers 16a and 16b are arranged. A recording paper 17 is transported between the transfer unit 15 and the carriage 11 by means of the transporting rollers 16a and 16b.
The process unit 12 of the carriage 11 is equipped with an image carrier 21, which is rotated at a circumferential speed synchronized with the movement of the carriage 11. The surface of the image carrier 21 is uniformly electrified by a charger 22. An electrostatic latent image is formed on the surface of the image carrier 21 by an exposure unit 23. The electrostatic image is visualized as a toner image by means of a developing roller 24. The toner image is transferred onto the recording paper 17 by means of the transfer unit 15 located at the side of the recording paper 17 opposite to the image carrier 21. The toner particles remaining on the image carrier 21 are scraped away by a cleaner 25.
The surface of the image carrier 21 which has been cleaned in the above way is electrified again by the charger 22, and the same process as described above is repeatedly performed. When the printing along a predetermined width is completed, the recording paper 17 is transported by a predetermined amount of movement by means of the transporting rollers 16a and 16b. The carriage 11 is returned to the predetermined position (home position), and performs the printing again. The home position is defined as a position which is located above the transfer unit 15 but is not located above the recording paper 17.
The image transferring by the transfer unit 15 is performed by applying a voltage to the transfer unit 15, an electrically conductive member 15b such as an electrically conductive rubber sheet is placed on a base 15a.
The fixing unit 13 is equipped with a fixing roller 26 in which a heat source 27 such as a halogen lamp is provided. The fixing roller 26 is preheated to a predetermined temperature by the heat source 27 before starting the printing operation. The temperature measured during printing is sensed by means of a temperature detector such as a thermistor (not shown for the sake of simplicity), and the fixing roller 26 is controlled based on the sensed temperature. That is, the fixing unit 13 is moved together with the process unit 12, and performs the fixing operation immediately after the image transfer by the process unit 12.
FIG. 2 is a diagram of another conventional carriage. In FIG. 2, parts that are the same as those shown in the previously described figures are given the same reference numerals. A halogen lamp (infrared lamp) 28 is provided, as a heat source, in the fixing unit 13 of the carriage 11. Further, a mirror 29 is provided in the periphery of the halogen lamp 28. The fixing unit 13 directly applies heat to the recording paper 17 in order to fix the image transferred to the recording paper 17 thereto.
However, the conventional carriages 11 shown in FIGS. 1A through 1C and FIG. 2 have the following disadvantages. After images are recorded on the recording paper 17, the carriage 11 is returned to the home position. That is, the carriage 11 waits for the starting of the next printing process at the position outside of the recording paper 17. In this state, the fixing roller 26 shown in FIGS. 1A and 1B is in direct contact with the electrically conductive member 15b of the transfer unit 15, and the halogen lamp 28 shown in FIG. 2 directly projects heat on the electrically conductive member 15b. Hence, there is a possibility that the electrically conductive member 15b may be damaged. Further, the long preheating time is needed to heat the fixing roller 26 to the predetermined temperature because heat is radiated from the fixing roller 26 via the transfer unit 15.
The fixing unit 13 shown in FIGS. 1A and 1B needs the halogen lamp 27 of a large size. Hence, the size of the fixing roller 26 is large. Further, the temperature distribution along the axis of the fixing roller 26 may be uneven due to the filament density of the halogen lamp 27.
Further, in the direct heat projecting by the halogen lamp 28 shown in FIG. 2, the energy density of the halogen lamp 28 is low and the rate of fixing toner images to recording paper is low due to the non-contact fixing process. Hence, it takes a long time to complete the fixing process.
As a disadvantage common to the carriages 11 shown in FIGS. 1A through 1C and FIG. 2, there is a possibility that the temperature of the carriages 11 may be increased to an abnormal value due to a fault of the temperature detector or the like if the carriage 11 stops operating due to a jam occurring in the printing process.